1. Field of the Invention:
This invention relates generally to an amplifier with separate AC and DC feedback loops which is suitable for use as a first stage amplifier for a reproduced signal by a record player in which both of a moving coil type cartridge and a moving magnet type cartridge can be used, and is directed more particularly to an amplifier with separate AC and DC feedback loops which can produce a stable output signal regardless of the variation of a power source voltage.
2. Description of the Prior Art:
In general, the reproduced output signal voltage from a moving coil type cartridge is smaller than the reproduced output signal voltage from a moving magnet type cartridge by more than 20 dB. Therefore, in a record player which can use both of the moving coil type cartridge and moving magnet type cartridge, it is necessary that when the moving coil type cartridge is used, a low noise head amplifier for the moving coil type cartridge be used. The use of such a low noise head amplifier results in the record player becoming complicated in construction, high in cost, with a disadvantage being caused in view of the maximum input voltage being required.
To avoid this disadvantage, in the art there has already been proposed a circuit as shown in FIG. 1, in which by changing the gain of an equalizer amplifier the output signal from both the moving magnet and coil type cartridge can be processed and at the same time the gain of an open loop can be changed. This prior art circuit will be now described with reference to FIG. 1. In FIG. 1, an input terminal 1, which is supplied with the reproduced output signal from the moving coil or magnet type cartridge, is connected through a DC blocking capacitor 2 to the base of an NPN-type transistor 3 whose base is in turn grounded through a resistor 4. The transistor 3 is connected at its collector through a resistor 5 to a positive power voltage source terminal 6, which is supplied with a positive DC voltage +Vcc, and at its emitter is connected through a resistor 7 to a negative power voltage source terminal 8 which is supplied with a negative DC voltage -Vcc whose absolute value is equal to that of positive DC voltage +Vcc. The collector of the transistor 3 is connected also to the base of a PNP-type transistor 9, which is connected at its emitter to the positive voltage source terminal 6 through a parallel connection of a resistor 10 and a capacitor 11 and at its base to its collector through a capacitor 12. The collector of transistor 9 is connected through a capacitor 13 to an output terminal 14. The collector of transistor 9 is connected also to the emitter of transistor 3 through a feedback element 15 for obtaining the characteristics of an RIAA standard and also to the drain of a field effect transistor (which will be hereinafter referred to simply as an FET) 16 which will form a constant current circuit and also increase the gain of transistor 9. This FET 16 has the source connected through a resistor 17 to the negative voltage source terminal 8 and the gate connected to the negative voltage source terminal 8. The connection point between the emitter of transistor 3 and feedback element 15 is grounded through a series connection of a capacitor 18 and a resistor 19, and the connection point between capacitor 18 and resistor 19 is grounded through a series connection of a resistor 20 and a connection switch 21. In this case, it is selected that a resistance value R2 of resistor 7 be a great deal larger than that of R3 and R4 of resistors 19 and 20 and the ratio between resistance values R3 and R4 is selected to be 9:1. Further, in this case, the input impedance of the transistor 9 is selected much greater than a resistance value R1 of resistor 5.
With the prior art amplifier circuit of FIG. 1, when a moving magnet type cartridge is used, the connection switch 21 is open (i.e., OFF). At this time, the gain of transistor 3 becomes about R1/R3. When a moving coil type cartridge is used in the circuit of FIG. 1, the connection switch 21 is closed (i.e., ON). At this time, the gain of transistor 3 becomes about 10R1/R3 because R3:R4=9:1. Thus, when either one of the moving magnet and coil type cartridges is used, the circuit of FIG. 1 delivers an output signal with the same level to the output terminal 14. In the circuit of FIG. 1, when the connection switch 21 is closed, the gain of the closed loop increases by about 20 dB and also the gain of the open loop increases by about 20 dB. Thus, at this time, the negative feedback with the amount the same as that of the negative feedback upon the connection switch 21 being open is provided and hence the amplifier circuit of FIG. 1 has good stability.
However, in the circuit of FIG. 1, when the DC voltage -Vcc applied to the negative voltage source terminal 8 fluctuates, this fluctuation appears at the output terminal 14 as the resistance value R2 of resistor 7 relative to the impedance value of negatve feedback element 15 for the fluctuation. Further, it is necessary that the voltage at a connection point P between the collector of the transistor 9 and the drain of FET 16 is kept at the mid voltage between the voltage applied to the positive and negative voltage source terminals 6 and 8 or at zero volt in this example. However, the voltage at the connection point P is moved by the fluctuation or scattering of DC voltage -Vcc applied to the negative voltage source terminal 8 with the resistance value R2 of resistor 7 for the impedance value of feedback element 15, and hence it is difficult for the voltage at the connection point P to be kept at the mid voltage between that applied to the terminals 6 and 8, for example, zero volt.